US3403048A - Heat-sealable, moistureproof coating compositions and coated packaging material incorporating same - Google Patents

Description

United States Patent 3 403,048 HEAT-SEALABLE, MbISTUREPROOF COATING COMPOSITIONS AND COATED PACKAGING MATERIAL INCORPORATIN G SAME James H. Grimm, Milford, N.J., and Francis J. Wieloch, 5

Penfield, N.Y., assignors to Riegel Paper Corporation, New York, N.Y., a corporation of Delaware No Drawing. Continuation-in-part of application Ser. No, 336,885, Jan. 10, 1964. This application May 3, 1967, Ser. No. 635,667

6 Claims. (Cl. 260-3) ABSTRACT THE DISCLOSURE Cross reference to related applications.-This application is a continuation-in-part of copcnding application Serial No. 336,885, filed January 10, 1964, for HeatSealable, Moistureproof Coating Compositions and Coated Packaging Material Incorporating Same, now abandoned.

Summary of the invention.-In the packaging of food products, for example, it is often important for the wrapping or packaging material to have predetermined barrier properties, as regards the transmission of water vapor and gas, either into or out of the package. In addition, the packaging material desirably should be suitable for formation and closure by heat-sealing techniques, under circumstances which will provide for a high-strength seal under a fairly wide range of operating conditions, such as may be experienced in regular production operations on various form and fill packaging machines. These and additional characteristics desirable and acceptable in a commercial packaging material are imparted to packaging substrates by the new and improved compositions of the invention. Among other beneficial characteristics of the new coating are good gloss, adequate scufr' resistance, sufficient flexibility, resistance to blocking, ability to accept pigments, clarity, ability to form stable solutions, etc. Various coated packaging materials now available for practical commercial use, while having some of the desirable characteristics sought in an ideal packaging material, are substantially inadequate in other respects. Particularly, it has been difficult to achieve desired moisturcproof and heat-seal properties in a form and fill packaging material of acceptable cost without introducing ingredients which would be undesirable for food packaging use.

As an important aspect of the invention, the new coating composition incorporates, as a principal film-forming constituent, relatively highly saturated cyclicized rubber, advantageously a cyclicized natural rubber having an iodine value of approximately 95 or less. The highly saturated cyclicized rubber, which most advantageously constitutes about 50 percent of the new composition, is modified by the addition of wax, for improved water vapor transmission rate (WVTR) properties, and by the addition of a vinyl or similar modifier for improved heat-seal characteristics. In one particularly advantageous composition, the wax modifier constitutes about percent 3,403,048 Patented Sept. 24,4968

ice

of the composition and the heat-seal modifier constitutes about 30 percent of the composition, although tliese percentages will, of course, vary somewhat with the characteristics of the specific modifiers. (References herein to percentages or parts of constituents means percentage or 'parts by weight, unless otherwise specified.)

A glassine substrate coated with the composition of the invention exhibits markedly superior WVTR and gas barrier charasteristics, while at the same time providing fiber tear heat seals at extremely low coating weights. These primary characteristics, coupled with additional desirable properties, make the composition particularly suitable and advantageous for use in conjunction with the package of foods.

More specifically, the present invention provides, in a single surface coating, both adequate fhot tack and adequate non-sticking properties for sheet materials especially adapted for use in the rapidly developing field of form and fill packaging. As is known in the art, a form and fill machine is a high speed, automatic, bag former and filler which generally operates by overfolding a continuous web and continuously forming a longitudinally extending seam to define a continuous tube. Thereafter, the tube is transversely sealed, filled through the open end, advanced, and then sealed transversely. Each transverse seam forms, in elfect, the top of one package (a filled package) and the bottom of the next package to be filled.

In order to be operated at economically feasible, very high speeds, the sealing jaws of the packaging machinery must remain uncontaminated from the coatings of the packaging material and must be able to effect, without an unnecessarily long dwell, a seal having suflicient hot tack. It should be understood that a freshly formed bag seam with sufficient hot tack will remain cohesive and intact under the stress of the packaged contents while the adhesive is setting (tacky) and after the sealing jaws have been withdrawn.

In previously known coating compositions, acceptable hot tack properties have been obtained at the expense of the non-stick properties and vice versa. That is to say, the coatings of the prior art possess either sufiicient hot tack or suificient non-stick properties but none possesses a high degree of both, as is most desirable for use in very high speed form and fill equipment.

Specifically, it has been discovered that an ethylenevinyl acetate copolymer is an especially efiective heat seal modifier for use with a highly saturated cyclicized rubber and a high melting point wax to provide a new and improved coating composition having unexpectedly superior form and fill properties, i.e., high hot tack and high resistance to sticking and having superior resistance to water vapor transmission. Moreover, it has also been determined that these extraordinary coating compositions, simultaneously having highly desirable anti-sticking and hot tack properties, as well as an extremely low WVTR, may be readily formed into solvent solutions and coated upon printed substrates to form clear, transparent barriers having good gloss.

The particular cyclicized rubber compound employed to great advantage in the practice of the invention has a very high melting point (at least 125 C.) and a low iodine value (-95) in comparison with the conventional Pliolitc type cyclicized rubbers employed heretofore in packaging coating compositions of the prior art. More specifically, cyclicized rubbers for the practice of the invention may be prepared by carrying a reaction between an inorganic acid and a rubber to substantial completion to yield a substantially completely saturated conversion product. In significant contrast, the Pliolitc type of cyclicized rubbers are comparatively less than substantially completely saturated, and are usually prepared by a. controlled process which-results in a substantially ingop plete reagtionof therubber and acid. The non-equivalence of the two types of cyclicized rubbers is clearly evidenced by significant differences in melting points and iodine values.

In a most advantageous specific embodiment of the invention, the main constituent of the composition is a cyclicized natural rubber, as made available as of January 10, 1964, the filing date of the aforementioned copending application Ser. No. 336,885, under the trade designation RCI 92-154, by its manufacturer, Reichhold Chemicals, Inc. RCI 92-154 is a hard, tough, hornlike, resinous polymer which is prepared by carrying to substantial completion the reaction between. rubber and an inorganic acid in the presence of a phenol catalyst, and which has the following approximate properties and characteristics as specified by the manufacturer, Reichhold Chemicals, Inc.:

Specific gravity: 25 C. 1.00 Wt. per gal.-25 C 8.33 Bulking value25 C. 0.120 Index of refraction-25 C. 1.535 Color, U.S.D.A., Rosin scale M Melting point, B & R method, "C 145-165 Melting point, capillary method, C 125-135 Molecular weight, average 5,000-10,000 Iodine value 75-95 Acid value -1 Saponification value 0 Burning rate Very slow Dielectric constant, 1000 cy. 1.35-1.50 Specific surface resistance, ohm Hardness, Sward 50-60 The specific portion of RCI 92-154 in the new and improved coating composition can be varied, as will be understood, however, the best WVTR characteristics are realized when the percentage of RCI 92-154 is above about percent and less than about 70 percent, and the very best, indeed optimum, results are realized when the percentage of RCI 92-154 is approximately percent (i.e., above 40 percent and less than percent).

It has been determined that the more unsaturated types of cyclicized rubbers (e.g., Pliolite resin P-1230) are unsatisfactory in the new compositions and are clearly nonequivalent resins for the purposes of practicing the in vention described in this application. Specifically, experiments using Pliolite resins (which have high iodine values), certain phenol reacted products, other resins including phenolic modified rosin (e.g., maleic anhydride modified rosin), and the like as substitutes for the RCI type resin in the new compositions failed to provide coating compositions having comparable properties to those of the coating compositions in which the RCI 92-154 cyclicized rubber was employed. For example, the new compositions employing RCI 92-154 resin may be readily dissolved in toluene or a similar solvent to establish a stable, working solution which may be applied to substrates by conventional techniques and which will dry to form a clear coating. However, when a more unsaturated rubber (e.g., a Pliolite P-1230 resin) is substituted for the specified highly saturated rubber (RCI 92-154), it is impossible to form a stable, working solution (i.e., one which will completely dissolve as a clear, non-separating solution and which will dry as a clear coating) in toluene or any other solvents.

To illustrate further the critical importance of the RCI 92-154 type resin in the new composition, formulations substituting another well known film-forming constituent (maleic anhydride modified rosin, available from Reichhold Chemical Co. as Beckacite 1110) for the RCI 92- 154 resin were prepared and tested for hot tack. The new coating compositions and the compositions containing the Beckacite 1110 were identical, but for the substitution of Beckacite 1110 resin for the RCI 92-154 resin. The results of these tests proved that other resins (such as a maleic anhydride modified rosin) when em ployed ,in lieu of the specified highly saturated cyclizecl rubber do not provide comparable compositions in terms of hot tack, etc.

Through the aforementioned experimentation, including testing and evaluating heat seal coating compositions which included various cyclicized rubber resins prepared from both natural and synthetic rubbers, it has been discovered that only resins, of which the resin RCI 92-154, manufactured by Reichhold Chemicals, Inc. is representative, are effective in providing a composition having the requisite properties of hot tack, anti-stick, low water vapor transmission, etc., when combined with modifiers such as ethylene-vinyl acetate copolymers and parafiin wax.

As mentioned hereinabove, one of the significant modifiers utilized in the composition of the invention is wax, and in a specifically preferred composition, the wax employed is parafiin having a melting point of 136-140 F. and sold by the Sun Oil Company under the designation Sun 4312 Paraffin Wax. Microcrystalline and vegetable waxes can be used with acceptable results, although parafiin wax is much preferred for the better WVTR characteristics it imparts to the composition. Waxes of lower melting point than the specified range, While providing an improvement in WVTR characteristics, tend to lessen the resistance of the composition to blocking. Higher melting point ranges, on the other hand, while improving resistance to blocking, tend to reduce WVTR prop erties, particularly as measured on coated substrates in a creased condition. Thus, while the particular Wax may be chosen in accordance with specific end use requirements, the specified wax provides an optimum balance for general use.

A second significant modifier utilized in the new compositions is a constituent imparting heat scalability. The most advantageous group of heat-sealing modifiers are vinyls and vinyl copolymers, although certain other compatible heat-sealable compositions have been employed. Perhaps the most advantageous specific heat sealing modifier is a vinyl acetate-ethylene copolymer manufactured by E. l. du Pont de Nemours & Co., Inc., under the trade designation Elvax 250. Based upon experimental results, it appears that the heat-sealing modifier may be employed in the new composition in amounts ranging from about 3 percent to about 65 percent, although it appears that optimum results are achieved using about 30 percent.

In addition to the beforementioned major components of the new compositions, it is desirable in certain instances to utilize minor quantities of additional additives such as anti-oxidants, ultraviolet absorbers, pigments, gloss controllers, mold release agents, and the like. The basic composition appears to be compatible with a Wide range of such minor additives. Suitable anti-oxidants are compositions of butylated hydroxy toluene or butylated hydroxy anisol. A small quantity (e.g., 1-3 percent) usually is adequate. For non-food uses, an ultraviolet absorber, such as Uvinol 400, a substituted benzophenone, may be used instead of or in conjunction with an anti-oxidant.

The composition of the invention may be readily dissolved in and is most readily applied to a substrate from an aromatic solvent solution. Typically, the substrate is coated upon one or both surfaces with the solvent being evaporated to form a continuous, clear, glossy and solid film. The common aromatic hydrocarbons (for example, benzene or toluene) have been found to be suitable solvents, and naphtha may also be utilized Where expedient. Typically, the solvent solution comprises on the order of to percent of the solvent and 30 to 20 percent of the new composition.

In a typical application, a glassine substrate web of about 30 pounds basis weight is coated on each side with about 1.5 to about 3 pounds per ream (3,000 square feet).

v 1 A coated substrate is considered to have good blOllklIlg resistance when two sheets, held together under psi. at F. for 24 hours, show no tendency to tick 01' mar upon separation.

However. where WVTR requirements are of "relatively less significance, lower coat weights can be utilized. Fiber tear heat seals are possible using coat weights as low as 0.6 pound per ream, although a more practical lower limit for commercial operations is about 1.5 pounds per ream. I

i In addition to the heat-seal modifiers referred to above, it has been determined that the following commercially available compositions (in addition to other vinyls, acrylics, styrenes, and their copolymers, as well as other rosin 5 esters and other wood-derived resins) will apparently form useful heat-seal modifiers: Polyisobutyl methacrylate, manufactured by E. I. du Pont de Nemours & Co., Inc., under the trade designation Lucite 45; acrylateethylene copolymers, manufactured by Union Carbide Co., under the trade designation Bakelite DQ DA2100 and Bakelite DQ DA3270; vinyl acetate-vinyl chloride copolymers (13% vinyl acetate), manufactured by Union Carbide Co., under the trade designation VYHH; vinyl acetate-ethylene copolymers (18% vinyl acetate), manu- *factured by E. I. du Pont de Nemours & Co., Inc., under TABLE II RCI Elvax 250 Sun 4312 Coating 92-154 (phr.) paraffin Heat-seal 2 WVTR 1 weight wax (1 100 63.6 0 Outstanding 6.22 2.95

" Per hundred parts rubber. For footnotes 1 and 2 see Table I.

TABLE I' Formulation:

RCI 92-154, 46%. Sun 4312 parafiin wax (melting point 140 F.). Heat-sealing modifier (as identified below), 29%.

Heat-seal characteristics 1 Good.

Heatsealing modifier WVTR 1 XYHL Polyvinyl Butyral, manufactured by Union Carbide Co. AYAF Polyvinyl Acetate (softening point 77 0.), manufactured by Union Carbide Co.

AYAT Polyvinyl Acetate (softening point 87.5 C.), manufactured by Union Carbide Co.

Elvax 250 Vinyl Acetate-Ethylene Copolymer (28% vinyl acetate), manufactured by E. I. du Pont de Nemours & Co., Inc.

Elvax 150 Vinyl Acetate-Ethylene Copolymer (33% vinyl acetate), manufactured by E. I. du Pont de Nemours & Co., Inc.

Elvax 3602 Vinyl Acetate-Ethylene Copolymer vinyl acetate), manufactured by E. I. du Pont de Nemours & Co., Inc.

Vinylite VMCH Vinyl Acetate- Vinyl Chloride Copolymer (13% vinyl acetate), manufactured by Union Carbide C0.

Vinylite VAGH Vinyl Acetate- Vinyl Chloride Copolymer (3% vinyl acetate), manufactured by Union Carbide Co.

Ester Gum 8-L Glycerol Ester of Rosin, manufactured by Hercules Powder Co.

Dow 276 V-2 poly-alpha-rnethyl styrene, manufactured by Dow Chemical Co.

1 WVTR determined by TAPPI Method T 464 M-45, reported in units of grams per 100 square inches per 24 hours at 95% relative humidity, 98 F. Figures reported throughout are for flat" sheets unless specified as being for creased sheets.

2 Indicated heat-seal readings are determined as follows: Outstanding: Yields fiber tear seals on paper over a wide range of temperatures including l70350 F. Seals with short dwell times'and low contact pressures (thus assuring good performance on high speed equipment). Has the ability to bear 15 gram/inch load at sealing temperatures. Requires 1 pound/ream or less coating to obtain a fiber tear seal to uncoated glassine 0. 40 Fair to good.

0. 05 Excellent.

0. 28 Fair to good.

0. l3 [Exceptional gloss] paper.

Excellent: Has the same qualities as Outstanding except that the ability to bear a load at sealing temperatures is reduced to 5-14 grams] the trade designation Elvax 420 and Elvax 460; and polyvinyl acetate, manufactured by Union Carbide Co., under the trade designation AYAF, plus low molecular weight polyethylene, such as manufactured by Allied Chemical Corp., under the trade designation AC-6.

In order to establish optimum ratios of wax to the cyclicized rubber and heat-seal modifier constituents, a

. series of tests was made utilizing 100 parts of ROI 92-154,

63.6 parts of Elvax 250, and zero to 327 parts of Sun 4312 Parafiin Wax. The various compositions were applied to both surfaces of a 30-pound plassine substrate, at coat weights ranging from 2.4 pounds per side per ream to 3.2 pounds per side per ream. Heat-seal characteristics and WVTR properties were determined in each instance and are indicated above in Table II.

From the foregoing table, it can be observed that heatsealing properties vary as an inverse function of the proportion of parafiin wax employed, while the WVTR properties vary on a direct relationship with the amount of wax. Highly satisfactory results are realized when the amounts of paraffin wax are in the range of 36 percent of the rubber to about 73 percent of the rubber, since all of the coated substrates in this range had heat-seal properties characterized as Excellent and WVTR values of less than .10. Moreover, depending upon the specific end use, compositions including lesser or greater amounts of Wax might be suitable.

A glassine substrate coated on both sides according to item number 6 of Table II exhibited highly superior gas barrier properties. By way of example, the coated glassine has an oxygen transmission rate of about 0.03 (cc./ sq. in./mil./24 hrs./1 atm., at 75 F., 0% humidity) as compared to the transmission rate of polyvinylidene chloride coated cellophane (at 35% relative humidity), which ranges from 0.5 to 1.5.

Where especially good heat-seal characteristics are desired, and WVTR is less important, wax can be reduced, proportionately, and a composition consisting of, say, 100 parts RCI 92-154, 63.6 parts Elvax 250, and 18.2 parts wax, applied to a typical fibrous substrate (e.g. glassine), will provide a fiber tear seal with very low coat weights.

Utilizing a constant, intermediate amount of paraffin wax, a series of tests was run in which the cyclicized rubber-to-heat seal modifier ratio was varied. This series addition to the foregoing, fiber tear goals were b i of tests is reflected in Table III below. with coat weights as low as .6 pound per side per ream.

TABLE III RCI Elvax 250 Sun 4312 Coating 92-154 ercent) paraffin Heat-seal 2 WVTR 1 weight (percent) wax (percent) For footnotes 1 and 2 see Table I.

The results indicated in Table III reflect that acceptable The composition of the invention, when applied to a heat-seal properties are realized over a wide range of suitable packaging material substrate, such as glassine, ratios of RCI 92-154-to-Elvax, holding the wax content provides a packaging material with outstanding WVTR constant at about percent. and heat-seal properties in addition to other characteristics At the lower levels of RC1 92-154, it was possible to which make the material highly desirable for food packrestore the WVTR properties to desirably low values by aging and other applications. While providing fiber tear significantly increasing the amounts of wax. This was done h at Seals, the new composition enable WVTR ratings of at some sacrifice to the heat-seal properties, although even 25 0.01 flat, 0.12 creased to be achieved. And in this respect, at as high as about 65 percent wax content an adequate it should be noted that WVTR flat ratings ten times as heat-seal was obtained, which was much better than would high as the last-mentioned value still would be considered be obtained With, for ex p Plain WaX 0f WaX modlfied exceptionally good from a commercial standpoint. By only wrth a vinyl acetatehyl p y' way of further contrast, available polymer coated cello- A particularly advantageous formulation for general apo phane fil used fig quently in packaging, have WVTR pllcatlon appe o be Parts RC1 92454: Parts ratings as high as 0.7 or about 70 times as high as that 512 17 1 g s j fi 'fti g i ggg sz i %g z obtainable with the optimum WVTR formulations accord- Pq HQ, an P 11 ing to the invention. Moreover, with the composition of coatrng was applied 1n we1ghts of approximately]; 2.7 to 360 the invention WVTR ratings we b 610W the 0 1 Pounds Per slde per ream to a vanety of Sn strates y tionally good) rating are readily obtainable (along with sol-vent coating techniques. Determinations were then made for adhesion of the coating composition to the substrate, fiber tear heat seals) under Productlon HdItIOHS using WVTR, heat-seal characteristics, and clarity. The results less than P P Side P Team 1 -pound glassine. are reported in T b1 IV Of particular slgnrficance as regards the commercial By way of further example, but not of limitation, other 40 use of the new Coating Composition for food Packaging satisfactory substrates are films of styrene, nylon, Saran, applications and the like is the fact that the composition, and tetrafluoroethylene. in addition to providing particularly good WVTR and TABLE IV Substrate Adhesion WVTR Heat-seal 2 Clarity Glassine Very good 0.02 Excellent Bleached kraft paper high o-do 0. 19 do quality. Bleached kraft paper low do For footnotes 1 and 2 see Table I.

In order to establish the overall heat-seal characteristics heat-seal characteristics, provides very good clarity and of a typical formulation of the composition of the invenis free of undesirable odors and resistant to blocking; tion, various weights of a given formulation were applied It Will be understood that the exact formulations mento a bleached kraft substrate. The formula in question was tioned herein are representative only, and reference should 100 parts RCI 92-154, 63 parts Elvax 3602, and 54.5 be made to the following appended claims in determining parts Sun 4312 Paraflin Wax. At a typical coat weight the full scope of the invention.

of 3 pounds per side per ream, fiber tear seals were made We claim:

at heat-seal temperatures ranging from 160 F. to over 65 1. A flexible packaging material having properties of 375 F. With temperatures in excess of 200 F., the minhigh resistance to water vapor transmission, heat sealimum dwell time was less than /2 second, and tentative ability, hot tack and jaw release, comprising:

seals were obtained using contact pressure down to as (a) a substrate coated with a coating composition low as 6 p.s.i. (the lower limit of the test equipment). adapted to contribute said properties;

Hot tack was particularly good; a bleached kraft substrate (b) aid composition including a i i l fil coated 3 pounds per side per rea-m sealed coating-to-coatforming constituent, a highly saturated cyclicized ing was able to support 25 grams per inch at 225 F. In rubber having an iodine number not in substantial excess of and having a melting point of at least 1 Elvax 3602 (designation of January 10, 1964) is available approximately C as of the filing (late of this application as Elvax 40, from Du Pout (c) said highly saturated cyclicized rubber being present in amounts ranging from approximately 25- 70% by weight of said composition;

((1) the balance of said coating composition including high melting point parafiin wax and a heat scalable material selected from the group consisting of polyvinyl acetate, polyvinyl butyral, copolymers of vinyl acetate-and ethylene having approximately 18-40% vinyl acetate, copolymers of vinyl acetate and vinyl chloride having approximately 3-13% vinyl acetate, glycerol esters of rosin, polystyrene, polyisobutyl methacrylate, and low molecular weight polyethylene;

(e) said high melting point paraffin wax being present in amounts ranging from approximately 18-65% by weight of said composition; and

(f) said heat scalable material being present in amounts from approximately 3-65% by weight of said composition.

2. A packaging material in accordance with claim 1,

in which said cyclicized rubber is further characterized as having the following properties:

Specific gravity-25 C 1.00 Wt. per gal-25 C. 8.33 Bulking value-25 C. 0.120 Index of refraction-25 C 1.535 Color, U.S.D.A., Rosin scale M Melting point, B & R method, C 145-165 Melting point, capillary method, C 125-135 Molecular weight, average 5,000-10,000 Iodine value 75-95 Acid value 0-1 Saponification value 0 Burning rate Very slow Dielectric constant, 1000 cy. 1.35-1.50 Specific surface resistance, ohm. 10 Hardness, Sward 50-60 3. A flexible packaging material in accordance with claim 1, in which (a) said heat scalable material is a copolymer of vinyl 10 acetate and ethylene having approximately 18-40% vinyl acetate.

4. A packaging material in accordance with claim 3,

in which (a) said cyclicized rubber is present in greater amounts than either of said wax and said copolymer; and

(b) said substrate is selected from the group consisting of glassine, bleached kraft paper, cellophane film, polyester film, polyethylene film, polypropylene film, aluminum foil, cellulose acetate film, polycarbonate film, styrene film, nylon film, Saran film, and tetrafiuor-oethylene film.

5. A packaging material in accordance with claim 3,

in which (a) said cyclicized rubber is present in amounts greater than 40 percent by weight of said composition and less than 60 percent.

6. A packaging material in accordance with claim 3,

in which (a) said cyclicized rubber comprises approximately percent by weight of said composition;

(b) said high melting point parafiin wax comprises approximately 20 percent by weight of said composition; and

(c) said copolymer of vinyl acetate and ethylene comprises approximately 30 percent by weight of said composition.

References Cited UNITED STATES PATENTS 2,376,778 5/1945 Kallander 260-734 2,413,432 12/ 1946 Carson 260-734 2,661,340 12/1953 Van Veersen 260-768 3,020,170 2/1962 Macauley 260-734 3,025,167 3/1962 Butler 260-285 3,189,573 6/1965 Oken 260-285 MURRAY TILLMAN, Primary Examiner.

M. TULLY, Assistant Examiner.